A mammalian pancreas is composed of two subclasses of tissue: the exocrine cells of the acinar tissue and the endocrine cells of the islets of Langerhans. The exocrine cells produce digestive enzymes that are secreted through the pancreatic duct to the intestine. The islet cells produce polypeptide hormones that are involved in carbohydrate metabolism. The islands of endocrine tissue that exist within the adult mammalian pancreas are termed the islets of Langerhans. Adult mammalian islets are composed of four major cell types, the α, β, δ, and PP cells. These cells are distinguished by their production of glucagon, insulin, somatostatin, and pancreatic polypeptide, respectively.
Diabetes mellitus results from the failure of cells to transport endogenous glucose across their membranes either because of an endogenous deficiency of insulin or an insulin receptor defect. Diabetes type 1, or insulin dependent diabetes mellitus (IDDM) is caused by the destruction of β cells, which results in insufficient levels of endogenous insulin. Diabetes type 2, or non-insulin dependent diabetes, is believed to be a defect in either the insulin receptor itself or in the number of insulin receptors present or in the balance between insulin and glucagon signals. Although diabetes runs in families, and a variety of heritable mutations have been implicated in the development of the disease, no single genetic marker has been identified that is responsible for this condition.
Current treatment of individuals with clinical manifestation of diabetes attempts to emulate the role of the pancreatic β cells in a non-diabetic individual. Individuals with normal β cell function exhibit precise regulation of insulin secretion in response to serum glucose levels. This regulation is due to a feedback mechanism that resides in the β cells that ordinarily prevents surges of blood sugar outside of the normal limits. Unless blood sugar is controlled properly, dangerous or even fatal levels can result. Hence, treatment of a diabetic individual involves monitoring of blood glucose levels and the use of injected bovine, porcine, or cloned human insulin as required. Despite such intervention, there is often a gradual decline in the health of diabetics.
Diabetes afflicts millions of people in the United States alone, and there is a clear need to provide cells capable of replacing pancreatic endocrine function. The ability to isolate distinct populations of live pancreatic endocrine cells represents a key step towards achieving this goal. This permits in vitro modeling of the Islet of Langerhans, for the study of normal and aberrant glucose metabolism and facilitate the isolation and/or evaluation of β cells. In addition, there is a need to produce new clinical treatments for diabetes, including the production of islet cells for transplantation (see U.S. Pat. No. 4,439,521; U.S. Pat. No. 5,510,263; U.S. Pat. No. 5,646,035; U.S. Pat. No. 5,961,972). Successful transplants of whole isolated islets, for example, have been made in animals and in humans. The success of the Edmonton protocol in the treatment of type 1 diabetes highlighted the promise of cellular replacement therapy for this disorder (Hirshberg et al., Rev Endocr Metab Disord. 4:381-389, 2003; Sharpiro et al., N Engl J. Med. 343:230-238, 2000). Unfortunately, insulin independence has not necessarily been shown to be durable in transplant recipients.
There is a need to identify and isolate islet cells or islet progenitor cells that can be used for β cell expansion or differentiation in vitro or for direct transplantation. Furthermore, there is a need for diagnostic methods that can accurately assess the number of pancreatic endocrine cells (or a subset thereof) in a subject, such as a subject with type 1 or type 2 diabetes.